Agni
- Appearance: Cephalopodic
- Two large, flexible spherical lobes serving as central mass (TOS Novel: The Captain's Oath)
- Resting underneath rigid, hinged hemispherical shells (TOS Novel: The Captain's Oath)
- Full body and most of tentacles can be pulled into shells for protection (TOS Novel: The Captain's Oath)
- Resting underneath rigid, hinged hemispherical shells (TOS Novel: The Captain's Oath)
- Thick tentacles used for movement, manipulation of environment (TOS Novel: The Captain's Oath)
- Horn-like structure projects forward from hinge of shells (TOS Novel: The Captain's Oath)
- Tipped with inflatable translucent sac (TOS Novel: The Captain's Oath)
- Inflates and deflates to serve as sensory organ (TOS Novel: The Captain's Oath)
- Tipped with inflatable translucent sac (TOS Novel: The Captain's Oath)
- Two large, flexible spherical lobes serving as central mass (TOS Novel: The Captain's Oath)
- Biological notes
- Native to Class N environment (TOS Novel: The Captain's Oath)
- Sulfuric acid-based biology relying on C-C bonds rather than C-O (TOS Novel: The Captain's Oath)
- Native to Class N environment (TOS Novel: The Captain's Oath)
- Historical notes
- First contact with Federation May 2262 (TOS Novel: The Captain's Oath)
- Attack in self-defense while evaluating class N world in Adelphous system for colonization (TOS Novel: The Captain's Oath)
- Further conflicts in Bardeezi system 2263, Regulus system 2264, during attempted colonizations (TOS Novel: The Captain's Oath)
- Diplomatic arrangements formed between Agni and Federation 226# following establishment of communications (TOS Novel: The Captain's Oath)
- First contact with Federation May 2262 (TOS Novel: The Captain's Oath)